Heat the butter over low heat in a large, heavy-based non-stick frying pan with an ovenproof handle.
Beat the eggs, milk, cornflour and seasoning together until frothy.
Gently pour into the preheated pan and cook over a moderate heat until slightly set.
Remove from stove top and grill under a preheated oven grill until egg starts to set.
Remove from oven grill, sprinkle cheeses over and grill for another 3 minutes until cheese has melted and egg has puffed up.
Remove from oven.
Using a large egg lifter, carefully fold omelette over and transfer to a plate.